1. Field of the Invention
The present invention relates to an electronic flash or strobe light unit having an insulated gate bipolar transistor (hereinafter referred to as IGBT for short) connected in series with a flash discharge tube so as to control the light emitting operation of the flash discharge tube, and more particularly to an electronic flash unit characterized by a voltage supply system for the flash discharge tube, which is effective in emitting light repeatedly at high speed.
2. Description of the Prior Art
A previously known electronic flash unit provided with an IGBT is disclosed in U.S. Pat. No. 4,839,686.
This conventional unit or apparatus has a circuit configuration as shown in FIG. 2. Specifically, it is composed of a high voltage DC power source 1 which is a well known DC-DC converter, a main capacitor 2 which is charged by the power source 1, a voltage regulating circuit 3 which is attached to the power source 1 and serves to supply a constant voltage to a light emission control circuit 7 described later, a known trigger circuit 4 for triggering a flash discharge tube 5, a control circuit 6 which is connected with a control means 8 within a camera body to transfer several kinds of signals and serves to produce several kinds of output signals such as a trigger signal for operating the trigger circuit 4, a light emission control circuit 7 for on-off controlling the IGBT connected in series with the flash discharge tube 5 to control the light emission from the flash discharge tube 5, and a double voltage circuit 9 for applying a voltage twice as high as a charging voltage to the main capacitor 2.
In operation, when switch Sw is turned on, the high voltage DC power source 1 operates. Then, the main capacitor 2 and a double voltage capacitor 9a are charged to have their polarity as shown by the high voltage outputted from the high voltage DC power source 1. Further, when switch Sw is turned on, a capacitor Ce which serves as a power supply for the control circuit 6 is charged by a low voltage power source E. Then, a capacitor 3a in the voltage regulating circuit 3 is also charged. Thus, the control circuit 6 starts to operate so that the light emission control circuit 7 enters a flash or light emission stand-by state.
In a state where the above respective capacitors have been charged, when the control means 8 in the camera body supplies a light emission starting signal to the control circuit 6, the control circuit 6 operates to produce a high level signal from its output terminal Oa, thereby turning on transistors Qa and Qb within the light emission control circuit 7.
When the transistors Qa and Qb turn on, the IGBT is turned on by the charged voltage in the capacitor 3a so that the trigger circuit 4 also operates. As a result, the flash discharge tube 5 consumes the charges stored in the main capacitor 2 to emit light
On the way of the above light emission, when the control means 8 supplies a light emission stopping signal to the control circuit 6, the control circuit 6 produces a high level signal from its output terminal Ob thereby to turn on transistors Qc and Qd in the light emission control circuit 7. Thus, the transistors Qa, Qb and IGBT which have been "on" turn off. As a result, the flash discharge tube 5 stops emitting light.
The operation as described above is the basic operation of the conventional electronic flash unit of FIG. 2.
The electronic flash unit provided with an IGBT as shown in Fig, 2 does not produce excessive light emission unlike another conventional flash unit which stops emitting light using a commutation capacitor, can perform the repetitive light emission at high speed and can also be miniaturized in its shape.
However, as a result of detailed examination of the high speed repetitive light emission operation, it was found that the following problem remains unsolved.
If the repetitive light emission operation for the electronic flash unit constructed as shown in FIG. 2 is performed at the cycle (e.g. a certain cycle band of several tens of Hz or more) higher than a predetermined cycle, the subsequent light emission will occur before the double voltage capacitor 9a is charged sufficiently. In this case, since the double voltage circuit 9 does not perform its essential operation, the flash discharge tube 5 cannot emit light. As a result, missing of the light emission will occur disadvantageously.
Specifically, the double voltage capacitor 9a will not be charged while the cathode potential of the flash discharge tube 5 is in a high level, but starts to be charged only when the potential becomes a low level.
It is well known that when the flash discharge tube 5 once emits light, even if energy supply therefor is stopped, the above cathode potential remains in a high level during the period from completion of the ionized state to restoration to the initial state. In addition, the double voltage capacitor 9a has a suitable charging time constant. For this reason, if the subsequent light emitting operation is performed during the above period or at a time when the above time constant does not still pass although the above period has passed, the double voltage capacitor 9a will not be charged sufficiently. The double voltage circuit 9 cannot therefore perform its essential operation.
Incidentally, with a very high cycle exceeding the above cycle, the subsequent light emitting operation will be executed in a state where the flash discharge tube 5 can emit light without being triggered. So the flash discharge tube 5 can emit light very easily, and thus missing of light emission does not occur. This is well known.
On the other hand, in order to miniaturize the flash discharge tube and increase the amount of light emission therefrom, the technique of increasing the inside gas pressure to give the discharge tube a high impedance was proposed. But, as well known, this technique increases the discharge starting voltage of the discharge tube. In addition, considering the high speed repeating light emission, the tendency of increasing the light emission starting voltage becomes further pronounced since the above miniaturization deteriorates the heat dissipation characteristic and the high impedance boosts the heat storage characteristic. Thus, the above malfunction of the double voltage circuit becomes more disadvantageous for the light emission from the flash discharge tube.